This invention relates to weighing scales and more particularly to hydraulic weighing scales intended primarily for medical office or home bathroom use to accurately indicate a person's weight by means of a convenient readout at eye level. As those skilled in the art are aware, many types of scales have been designed to satisfy this function but no one has designed and marketed a successful hydraulic or pneumatic type. This invention eliminates the technical mistakes of previous hydraulic scale designs by using a low pressure system, incorporating a positive low friction leveling mechanism, placing the weight readout at eye level, using a minimum number of simple parts, and using overall cost effective fabrication and assembly techniques.
The three most common personnel type scales on the present market are as follows:
(1) The spring and lever type with dial readout incorporated into the platform assembly.
(2) The spring and lever type with a potentiometer, electric circuit board, and battery operated electric digital readout in the platform assembly.
(3) The mechanical spring and lever operated or balanced beam unit with the readout at waist level.
The (1) spring and lever is the most inaccurate and the most popular probably because it is the least expensive. It consists of many parts all incorporated into the platform assembly and it has no calibration capability to any specific weight. All manufacturing tolerances (which are cumulative) are built into the assembly resulting in a scale that cannot be expected to indicate a person's weight accurately within several pounds, the amount of error varying from plus to minus for each assembly. The weight readout, which is in the platform, is too far away from a person's eyes and cannot be read easily, especially since it is sensitive to any motion of the person standing on the platform.
The (2) electro-mechanical type with electrical digital readout is a scale that has the same disadvantages as the (1) spring and lever type and has the additional disadvantages of requiring batteries to provide a lighted electrical readout and the inevitable replacement of these batteries at any inconvenient time. This type of scale will probably never use common household 115 volt alternating current because of building codes that disallow any electrical floor outlets in bathrooms that have bathtubs. These building restrictions are necessary due to the hazard that is present when an electrical appliance can fall into a bathtub as it is being used and electrocuting its occupant. This scale is also several times more expensive than the (1) spring and lever type. It has gained very low public acceptance.
The (3) spring and lever type or balanced beam type scale with a post to raise the weight readout to waist level is an improved version over the heavy five and one-half foot high scales still being used in hospitals and medical offices. This scale has had some success in replacing the larger scales in medical facilities but it will never be successful for consumer usage in the home because of its excessive size, weight, and cost. Additionally, most housewives would not consider it attractive enough for home usage.
Purely hydraulic units have tried to do the weighing job with one small flexible load cell which requires a high pressure gage. Thus one of the problems with purely hydraulic units has been too high an operating pressure. Additionally, tests have shown that a small load cell cannot transmit the desired load to the indicator to deflect it throughout its entire range. Small amounts of air entrapped in the fluid load path plus deflection of the bellows convolutions in an axial direction result in pressure losses that preclude proper deflection of the Bourdon tube assembly. When a weighing scale system involves high pressure it is also susceptible to developing leaks and it is too inaccurate due to the unnecessary high weight to deflection ratio. High pressure weighing scales would have a small fluid displacement so that calibration of the Bourdon tube assembly to indicate correct weight would be difficult if not impossible. In short, a small amount of liquid moved means high pressure and a small amount of deflection in the platform and load cell. Another problem is that most present bathroom scales do not adequately compensate for a load that is not centered properly on the platform. Their best accuracy depends upon centering of the load on the platform and this degree of load equalization is not always achieved nor can it be expected using the existing proposed methods.
This invention provides a mechanism that keeps the platform cover level in all planes regardless of off center load placement. It contributes a minimum of friction, and transmits the load equally into the top of the load cell. Patents which are of interest but not particularly pertinent to the instant invention except for U.S. Pat. No. 4,056,156 are U.S. Pat. Nos. 2,795,410, 2,516,545, 3,637,034, and 3,433,316.